Photovoltaic devices, photoelectric conversion devices or solar cells are devices which convert light, especially sunlight into direct current (DC) electrical power. For low-cost mass production thin film solar cells are being of interest since they allow using glass, glass ceramics or other rigid or flexible substrates as a base material (substrate) instead of crystalline or polycrystalline silicon. The solar cell structure, i. e. the layer sequence responsible for or capable of the photovoltaic effect is being deposited in thin layers. This deposition may take place under atmospheric or vacuum conditions. Deposition techniques are widely known in the art, such as PVD, CVD, PECVD, APCVD, . . . all being used in semiconductor technology.
A thin-film solar cell generally includes a first electrode, one or more semiconductor thin-film p-i-n or n-i-p junctions, and a second electrode, which are successively stacked on a substrate. Each p-i-n junction or thin-film photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer (p-type=positively doped, n-type=negatively doped). The i-type layer, which is a substantially intrinsic semiconductor layer, occupies the most part of the thickness of the thin-film p-i-n junction. Photoelectric conversion occurs primarily in this i-type layer.
Prior Art FIG. 1 shows a basic, simple photovoltaic cell 40 comprising a transparent substrate 41, e. g. glass with a layer of a transparent conductive oxide (TCO) 42 deposited thereon. This layer is also called front contact F/C and acts as first electrode for the photovoltaic element. The next layer 43 acts as the active photovoltaic layer and comprises three “sub-layers” forming a p-i-n junction. Said layer 43 comprises hydrogenated microcrystalline, nanocrystalline or amorphous silicon or a combination thereof. Sublayer 44 (adjacent to TCO front contact 42) is positively doped, the adjacent sub-layer 45 is intrinsic, and the final sub-layer 46 is negatively doped. In an alternative embodiment the layer sequence p-i-n as described can be inverted to n-i-p, then layer 44 is identified as n-layer, layer 45 again as intrinsic, layer 46 as p-layer. Finally, the cell includes a rear contact layer 47 (also called back contact, B/C) which may be made of zinc oxide, tin oxide or ITO and a reflective layer 48. Alternatively a metallic back contact may be realized, which can combine the physical properties of back reflector 48 and back contact 47. For illustrative purposes, arrows indicate impinging light.
Depending on the crystallinity of the i-type layer solar cells or photoelectric (conversion) devices are characterized as amorphous (a-Si) or microcrystalline (pc-Si) solar cells, independent of the kind of crystallinity of the adjacent p and n-layers. Microcrystalline layers are being understood, as common in the art, as layers comprising at least a Raman crystallinity of 15% of microcrystalline crystallites in an amorphous matrix.
Nowadays, efficiency of solar cells and low cost production are of increasing interest. Multiple-junction solar cells with at least two thin-film photoelectric conversion units stacked one on the other are highly efficient; however need increased effort in manufacturing equipment. Moreover, the deposition rate for microcrystalline silicon with PECVD methods in tandem (double-junction) solar cells is considerably lower than the rate of amorphous layers (under otherwise comparable conditions). Therefore, there is a need for low-cost high efficient amorphous silicon solar cells.
Today, thin film solar cells are heading for mass-production. Requirement for such mass production are integrated manufacturing processes, allowing efficiently and effectively manufacturing such cells. Yield, throughput, uptime, quality are ingredients to observe in such processes. On the other hand, a clear goal is to increase cell efficiency and other electrical properties of the solar cells. However, the so called Stabler-Wronski effect describes the tendency of amorphous silicon photovoltaic devices to degrade (drop) in their electricity conversion efficiency upon initial exposure to light. Light-soaking experiments which are internationally accepted, include exposing the test devices under AM1.5-like illumination at a controlled temperature of 50° C. during 1000 hrs. The stabilized performances are evaluated at standard conditions (AM1.5 1000 W/m2, 25° C.) and the relative efficiency degradation is given by the difference between the initial (before light-soaking exposure) and the degraded efficiency (after 1000 hrs) normalized to the initial efficiency. Therefore efficiency is not only important as an initial value but also as a stabilized value. Values of about 20% or more for the relative degradation as known in the art for a single-junction amorphous silicon cell with an i-layer thickness larger than 200 nm are a serious obstacle for the commercialisation of such PV cells.